The image disk is then glued to a piece of Walnut. The plywood prevents
the Walnut from cracking and breaking when dropped and the Walnut gives stiffness and mass
to the rim of the spindles. The Walnut wood is glued so it is cross-wise to the
grain of the Baltic for maximum whorl strength.

The undercut area of the 'Overlay' style removes mass from the center and
concentrates the remainder in the rim of the spindle where it is needed for a sustained
spin.

This is what the 'overlay' style and the 'rim' style whorls look like when
you cut them open.